A large application area for position-measuring devices, particularly angular measuring devices, is numerically controlled machine tools. They are used to determine actual positional data, which the numerical control requires to control the control loops by which the feed of the tool or the workpiece is controlled. To this end, they are generally coupled to the shaft of a motor, either directly or indirectly.
Malfunctions in the positional-data recording may cause major problems. One error source may be the failure of the coupling between motor shaft and position-measuring device, which results in the position-measuring device always providing the same position value. As a consequence, not only may it happen that the workpiece just processed becomes unusable since too much material is being removed, but also that the tool machine is damaged because, for example, a tool comes into contact with a rapidly rotating spindle. Even worse than the financial damage is the injury risk for the operators.
Such a malfunction may have different causes. One cause may be a break of the mechanical connection between the position-measuring device and the motor shaft connecting the angular measuring device to the motor spindle. Such a mechanical shaft failure leads to a standstill of the shaft of the position-measuring device, and the measured-position value no longer changes although the motor shaft is rotating.
An additional cause may be a failure in the electronic evaluation of the positional data in the position-measuring device, e.g., a malfunction in the transfer of the positional data into an intermediate memory. If, for example, there is a line failure in the control line that controls the storing of the positional data in the intermediate memory, the positional data stored in the intermediate memory no longer changes, so that the same position value is output over and over. Such a defect is also referred to as electronic shaft break or failure.
Both a mechanical and an electronic shaft failure are able to be detected in the numerical control during the positioning process in a rapid and reliable manner. This is not possible while a shaft is at a standstill, i.e., when an axle is to be held in a particular position. The reason is that the numerical control is unable to distinguish whether the drive of an axle does indeed not move or whether it is only the position value that does no longer change.
While a mechanical shaft failure during standstill, i.e., without a force acting on the shaft of the angular measuring device, is to be considered unlikely, an electronic shaft failure may occur independently of the feed movement. To detect an electronic shaft failure, conventional position-measuring devices often transmit analog track signals in addition to the digitally encoded position values. These are sin/cos signals which are generated according to the conventional principle of incremental position measuring. When evaluating the positional data in the numerical control, it is checked whether both the digital and the analog position values change. If this is not the case, for example, if the digitally encoded position values remain constant whereas the analog signals continue to change, the control recognizes the defect and is able to initiate appropriate measures.
The increased effort that may be required for the additional generation, transmission and evaluation of the analog signals is considered to be a disadvantage. This disadvantage may become even greater by the attempts to use only purely digital interfaces between the position-measuring devices and the numerical control.